Say "page", not "sheet", everywhere a user sees it

From the development repository at 854009c.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/introduction.md

@@ -4,7 +4,7 @@ Kokkoja lets people at different computers write together on a shared
 *page*.  Everyone in a session sees the same page.  Anyone can write on it
 by hand or type on it, and what one person writes appears on the others'
 screens as it is written.  A page can begin blank, or it can begin with an
-image, such as a scanned sheet or a photograph, which everyone then writes
+image, such as a scanned document or a photograph, which everyone then writes
 on.  Every page is saved automatically to a *notebook* on each person's
 own computer, so that after the session each participant keeps a complete
 copy of everything that was written.
@@ -33,10 +33,11 @@ layer of its own.  You can erase your own writing and nothing else; nobody
 else can erase yours.  Any layer can be hidden on your own screen without
 affecting anyone else's view.
 
-**A page that grows.**  A page is an A4 sheet, lying on its side.  When writing approaches an
-edge, the sheet grows in that direction, so you are never short of room.
-An image is placed at its natural size; one wider than the sheet is
-scaled down to fit, and the sheet grows downward to hold further images.
+**A page that grows.**  A page is A4 in size, lying on its side.  When writing approaches an
+edge, the page grows in that direction, so you are never short of room.
+An image is placed at its natural size; one wider than the page is
+scaled down to fit, and the page grows to hold an image taller than it
+or further images below.
 
 **Saved without a save command.**  The page you are on is written to your
 notebook continuously, as it changes.  Nothing is lost if a connection
@@ -72,8 +73,8 @@ Safari on an iPad or an iPhone, writing with a pencil or a finger.
 Kokkoja is a general tool for working on a page together at a distance:
 going through a document with a colleague, sketching a diagram while
 talking it over, correcting a draft, teaching anything that is done on
-paper.  Wherever two people would otherwise lean over one sheet, Kokkoja
-puts the sheet between their computers.
+paper.  Wherever two people would otherwise lean over one sheet of
+paper, Kokkoja puts that page between their computers.
 
 Its ideal use, and the one it was made for first, is teaching a foreign
 language over a video call.  The teacher puts a scanned exercise on the